Examining the Potential Contribution of the Hokusai Impact to Water Ice on Mercury

Author(s)

Carolyn M. Ernst, Nancy L. Chabot, Olivier S. Barnouin

Publication Year

2018

Journal

Journal of Geophysical Research: Planets 123: 2628–2646

DOI

10.1029/2018JE005552

Description

Key points:
• Hokusai is the best candidate source crater for Mercury’s water ice if it was delivered by a single recent impact event
• The Hokusai impactor is estimated as between 6 and 31 km in diameter, depending on the assumed projectile composition and impact velocity
• The Hokusai impact could account for the inventory of water ice on Mercury for impact velocities <30 km/s

Abstract: The polar cold traps of Mercury host an estimated 10^16–10^18 g of water ice. The relative purity of the water-ice deposits could indicate they were emplaced over a short time interval, and sharp albedo boundaries suggest that the water ice could have been emplaced relatively recently. Together, these lines of evidence are consistent with potential delivery by a single, large, young impact event. Hokusai is the most prominent large young impact crater on Mercury—if the bulk of Mercury’s water-ice inventory was indeed delivered by a single recent impact event, Hokusai is the best candidate source crater. This study constrains the impact conditions that created Hokusai from morphological and color observations of the crater and its ejecta. Results show that the Hokusai impact was recent and oblique. These parameters, coupled with retention and migration factors largely derived from existing lunar studies, are used to estimate the possible contribution of the Hokusai impact to water ice on Mercury. Assuming water-rich asteroidal or cometary impactors, the Hokusai impact event could account for the inventory of water ice on Mercury for impact velocities less than ~30 km/s, a velocity that is achieved by 24–32% of impacts into Mercury, depending on the size distribution model employed. The single impact delivery scenario therefore remains viable for Mercury. Robust modeling of a single large impact event on Mercury would supply critical insight into the feasibility of this scenario.

Plain Language Summary: There is abundant evidence that water ice resides in the permanently shadowed regions of Mercury’s poles, yet how it was delivered to the planet is unknown. One possibility is that the water ice was delivered by a single, large, young impact event. If this were the case, there would be evidence of this impact event in the form of a large rayed crater. Hokusai is the most prominent large rayed impact crater on Mercury—if most of Mercury’s water-ice inventory was delivered by a single recent impact event, Hokusai is the best candidate source crater. This study constrains the impact conditions that created Hokusai from observations of the crater and its ejecta. Results show that the Hokusai impact was recent and oblique. These conditions are used to estimate the possible contribution of the Hokusai impact to water ice on Mercury. Assuming water-rich asteroidal or cometary impactors, the Hokusai impact event could account for the inventory of water ice on Mercury for impact velocities less than ~30 km/s, a velocity that is achieved by a quarter to a third of impacts into Mercury. The single-impact delivery scenario thus remains possible for Mercury.

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